1. Field of the Invention
This invention relates to the manufacture of thin film devices, and more particularly to the reduction in the formation of hillocks in films of metal in such devices. Still more particularly, this invention relates to a method for reducing the formation of hillocks in aluminum layers in MOS devices and devices produced thereby.
2. Description of the Problem
When a thin metal film, particularly a soft metal such as aluminum with a high coefficient of thermal expansion, is deposited on a substrate with a low coefficient of thermal expansion, such as silicon, microscopic protrusions often appear in the surface of the film. Such protrusions commonly appear in the aluminum metallization layers deposited on oxidized silicon surfaces in the manufacture of integrated circuits. Protrusions appearing immediately after deposition of the aluminum film are termed "growth hillocks". The protrusions which develop after cycling the integrated circuit to a high-temperature during manufacturing steps are called "annealing hillocks".
In either case, these microscopic protrusions or hillocks are troublesome and can cause subsequent device failure. For example, hillocks can cause shorts between conductive layers in a device in areas where conductors cross over one another, or in elements of the device having two layers of conductor such as integrated capacitors. Hillocks can also cause device failure by causing gaps in the layers of photoresist applied to the device in patterning the metal layers. It is generally believed that hillocks form at metal grain boundaries as stresses caused by differences in thermal expansion coefficients between the metal film and the substrate are relieved during thermal cycling of the device.
Previous attempts to overcome the problem of hillock formation in thin metal films have included the addition of impurities such as silicon, copper, silver and gold to the aluminum to immobilize the grain boundaries in the metal film, see e.g. U.S. Pat. No. 4,012,756 issued Mar. 15, 1977 to Chaudhari et al. One suggested approach to reducing hillocks specifically in aluminum films is to treat the film to form a boehmite (AlO-OH) layer on its surface, see e.g. U.S. Pat. No. 3,986,897 issued Oct. 19, 1976 to McMillan et al.
Yet another recently disclosed approach to reducing hillocks in aluminum films is to alternate layers of aluminum and an aluminum oxygen alloy, by periodically introducing controlled bursts of oxygen into the aluminum deposition chamber. None of these above approaches has proven entirely satisfactory, however.
I have observed, during the manufacture of large scale integrated circuit devices, that in areas of the device where an aluminum film overlies a region of criss-crossing conductor lines that create a step-wise pattern of hills and valleys having a depth dimension of the same order of magnitude as the thickness of the film, hillock density (i.e. the number of hillocks per unit area) in the aluminum film is greatly reduced.